Even with a cheap body filler as the one your using (Bondo), you should be able to fill the holes without glaze. A better filler will have less of these holes, but you should still be able to handle it with the cheap stuff. Try mixing the hardener as quickly as you can and get it on there in less than 30 seconds. Also use very thin applications. Use a plastic spreader. Never mix bondo on cardboard. There is oil in cardboard that causes "pot holes." Use a piece of metal or plastic instead. Sand with 220 grit. I usually start with 80 grit but that's for major body work and I don't think you will need it for this app. Don't worry about water sanding because I'm sure your dash has some texture to it so there will be no need for a mirror surface.